The genetic study of mammalian RNA tumor viruses has been undertaken to determine the methods by which they cause cancer in infected host animals: (1) Murine sarcoma virus (MSV), which is a deletion mutant of its helper murine leukemia virus (MuLV), lacks the information necessary for viral reverse transcriptase and requires for production of progeny the functioning of helper virus genes supplying at least viral envelope antigen(s) (gp71) and virion core protein polymerase. (2) Isolation of MSV in nonproductively infected mink cells has permitted the development of a rapid focus induction assay for certain mammalian helper type C viruses.